Aquatic Toxicology 75 (2005) 127–135 Contrasting effects of chloride on the toxicity of silver to two green algae, Pseudokirchneriella subcapitata and Chlamydomonas reinhardtii Dae-Young Lee 1 , Claude Fortin, Peter G.C. Campbell ∗ INRS-Eau, Terre et Environnement, Universit´ e du Qu´ ebec, Institut National de la Recherche Scientifique, 490 de la Couronne, Quebec City, Que., Canada G1K 9A9 Received 17 January 2005; received in revised form 20 June 2005; accepted 29 June 2005 Abstract Short-term silver toxicity was determined for two freshwater algae, Pseudokirchneriella subcapitata and Chlamydomonas reinhardtii, in the presence and absence of chloride. Silver speciation in the exposure media was controlled and algal growth was measured over 6 h. For P. subcapitata, an alga with low Ag uptake fluxes, silver toxicity could be predicted on the basis of the free Ag + concentration, in the presence or absence of significant complexation by chloride ions, as predicted by the biotic ligand model (BLM). For C. reinhardtii, an alga with high Ag uptake fluxes, silver toxicity was better predicted by the concentration of all labile dissolved Ag species than by free silver, a result that is consistent with diffusion through the unstirred layer surrounding the cell surface being the rate-limiting step in silver uptake. For both species, growth inhibition could be predicted on the basis of the Ag intracellular quota in the presence or absence of chloride, indicating that silver toxicity is a direct result of intracellular accumulation rather than cell surface interactions. © 2005 Elsevier B.V. All rights reserved. Keywords: Biotic ligand model; Intracellular metal; Growth inhibition; Phytoplankton; Silver; Speciation 1. Introduction Freshwater animals exhibit a wide range of silver sensitivities with LC 50 values ranging from ng/L (pM) ∗ Corresponding author. Tel.: +1 418 654 2538; fax: +1 418 654 2600. E-mail address: peter.campbell@ete.inrs.ca (P.G.C. Campbell). 1 Present address: Wastewater Technology Centre, Environment Canada, 867 Lakeshore Road, Burlington, Ont., Canada L7R 4A6. to mg/L (M), and the profound influence of the speciation of silver on its toxicity to aquatic vertebrates and invertebrates is well recognized (Wood et al., 2002). Silver toxicity to algae, however, has been largely unexplored, despite the ecological importance of phytoplankton and their recognized role in the biogeochemical cycling of metals (Sigg et al., 1995). A literature search yielded only five EC 50 values for the effects of silver on freshwater algae, ranging from 24 nM (Pseudokirchneriella subcapitata, formerly 0166-445X/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aquatox.2005.06.011